This invention relates to improvements on an electron tube having a photoelectric screen, such as an X-ray image intensifier.
An electron tube having a photoelectric screen, for example, X-ray image intensifier or gamma ray image intensifier is generally applied to the detection of high energy rays such as X-rays and gamma rays. The input window material of these intensifiers has hitherto been formed of glass. Recently, however, the input window of these intensifiers has come to be formed of a thin metal sheet. The reason is that in view of technical improvement of processing metal and rise in the cost of raw materials, a metal input window will eventually assure lower cost, if consideration is given to the processes and materials involved in the completion of such intensifier. Further, application of a metal input window has led to the tendency of using metal to other parts of the intensifier.
The photoelectric screen is generally prepared from a reaction product of a semimetal such as Sb, Bi or Te and an alkali metal. The method of forming the photoelectric screen is broadly classified into two types from a difference in the vacuum deposition processes. One of said processes comprises holding an alkali metal source and semimetal source in an envelope. For example, Cs and Sb are alternately deposited for mutual reaction on the substrate of an input screen from the corresponding alkali metal source and semimetal source repeatedly, until the photoelectric screen obtains a maximum sensitivity. However, this process has the drawback of presenting difficulties in enabling a photoelectric screen to have a high sensitivity.
Another process comprises initially depositing a prescribed amount of a semimetal, for example, Sb on the substrate of an input screen, and later depositing an alkali metal such as Cs for reaction with said semimetal, thereby to produce a photoelectric screen. If the quantification of Sb can be effected with high reproducibility, then the latter process is effective to produce a photoelectric screen with high sensitivity and stability.
The quantification of Sb applied in the latter process has hitherto been carried out in the following manner. A photoelectric tube is set near a photoelectric screen and outside of that portion of an envelope, to the inner wall of which Sb can be deposited. A light source is provided on that side of an X-ray image intensifier which is opposite to that thereof which faces a photoelectric tube. The quantification of Sb has hitherto been effected by reading the extent to which photoelectric current flowing in the photoelectric tube varies with an amount of Sb deposited on the inner wall of the envelope. However, the conventional quantification process of Sb has the undermentioned drawback that said quantification process is subject to heavy limitation depending on the structure of an image intensifier.
Recently, the internal structure of that type of an electron tube having a photoelectric screen in which electron beams are focused by an electric field is increasingly complicated due to the demand for the high performance of the electron tube. Referring to, for example, an X-ray image intensifier, the conventional type comprising three electrodes has been changed into the type which is provided with four electrodes in order to make a view field variable, or into the type which is formed of five electrodes in order to improve the distribution of resolution. Thus, the recent trend of an electron tube goes toward the type having an increasing number of electrodes. With an X-ray image intensifier having the above-mentioned inner structure, the passage of a light emitted from a light source is obstructed by a plurality of electrodes held in said intensifier, making it impossible to adopt the Sb quantification process.
Further, an attempt to prepare an input window material and other parts of an envelope from metal has come to be more widely accepted in order to reduce the scattering of a light in input window of the X-ray image intensifier and decrease the material cost and work cost. With the X-ray image intensifier provided with a metal envelope, it is obviously impossible to adopt the aforementioned quantification process of Sb.